This invention relates to engine diagnostic apparatus and more particularly relates to apparatus for diagnosing an engine by disabling a predetermined component or cylinder thereof.
Automobile owners frequently complain that the engines of their automobiles run roughly or appear to "miss" on one or more cylinders. In order to diagnose the engine when such operating conditions are encountered, automobile mechanics have tried to discover the cylinder which is missing. In order to perform the disgnosis, many mechanics have found it useful to disable the cylinders of the engine one-at-a-time and to observe the resulting engine performance. If a properly operating cylinder is disabled, the engine runs more roughly, whereas if a defective cylinder is disabled, little or no change in the engine operation is observed. By systematically disabling the cylinders of the engine one-at-a-time, the mechanic may quickly locate the defective cylinder.
A variety of devices for performing the abovedescribed engine diagnosis have been described in the past. One such device is shown in U.S. Pat. No. 3,788,129 (Trussell -- Jan. 29, 1974). Another such device is shown in U.S. Pat. No. 3,573,608 (Marino -- Apr. 6, 1971) and U.S. Pat. No. 3,572,103 (Marino -- Mar. 23, 1971).
Each of these devices has exhibited deficiencies which have limited their overall usefulness. For example, the Trussell apparatus employs a counter in which different counting states represent different engine cylinders depending on whether four, six 6 or eight cylinder engines are being diagnosed. This arrangement complicates the decoding circuitry connected to the output of the counter and generally requires the use of mechanical switches in such circuitry. The Marino apparatus employs a ring counter which must be reset during each engine cycle by a signal from a reference spark plug. This complicates the circuitry required when the reference spark plug is the device being disabled during the engine diagnosis.
The applicants have invented an improved cylinder disabling system in which a bistable device is used to produce binary output signals having states representative of the cylinders of an engine. The bistable device and associated gating circuitry are arranged so that a particular output signal state always represents the same engine cylinder irrespective of whether a four, six or eight cylinder engine is being diagnosed. This feature enables the system to employ an all electronic decoding network of a simple and inexpensive design.
In addition, the applicants' apparatus uses a setting pulse from a reference spark plug to set the output signals to an initial state only during the initial start up of operation. Thereafter, the output signals automatically are returned to the initial state without the use of the setting pulse. This feature enables the bistable device to automatically disable the cylinder employing the reference spark plug without the necessity for additional complicating circuitry.
In addition to the foregoing features, the applicants' invention divides the repetition rate of the setting pulses before applying them to the bistable device. This feature provides additional diagnostic flexibility which is lacking in a device which depends upon a setting pulse to reset a counter during every engine cycle.